It has previously been proposed to construct fluorescent lamps in form of a U-tube, into which electrodes are secured at the free ends of the legs of the U, and then, after applying a fluorescent coating to the inside of the tube, evacuation and filling of the tube with a suitable fill of noble gas and mercury, sealing the tube by a pinch or press seal. A lamp of this type is described in the referenced U.S. Pat. No. 4,481,442, Albrecht et al, assigned to the assignee of the present application, the disclosure of which is hereby incorporated by reference. In this lamp, the base of the U has a very specific shape in order to provide cooling zones which control the mercury vapor pressure within the lamp.
In order to increase light output from mercury vapor discharge fluorescent lamps, it has also been proposed to couple a plurality of parallel straight lamps or lamp sections by coupling connections to form a single discharge vessel of the respective straight elements, see for example U.S. Pat. Nos. 3,501,662, Plagge, 4,374,340, Bouwknegt et al, and 4,530,710, Dullea et al. In order to provide for "cold spots" or cooling zones which control vapor pressure in the discharge lamp, the coupling connections have been placed at specific and predetermined distances from the respective ends of the straight tubular portions, as described in detail in U.S. Pat. No. 4,374,340.
Straight tubular portions, by suitable coupling, thus can be assembled together to form discharge vessels of various shapes, such as H-shaped discharge vessels, in which the cross bar connection of the H can be fairly close to an upper end portion of the vessels to enhance light output. Multiple arrangements are possible, that is, the cross connections between multiple adjacent tubes, such as three or four, can be placed, staggered, closer to the upper and lower ends of the respective straight tubes. Forming such coupling connections--of which always one less than the number of straight tube units are required--is comparatively expensive and requires precise adjustment of complex apparatus. The elements are cross-connected by melting together projecting collars of adjacent tubes. This requires highly precise positioning of the tubes with respect to each other so that the overall dimensions will be accurately maintained to permit the parallel tubes to, later on, fit into a base. The end portions of the cross connections experience a drop-off of light and thus do not present, to the viewer, a uniformly U-shaped illuminated structure but, rather, a plurality of longitudinal light bars of varying light intensity which, for some applications, is esthetically undesirable.
It has also been proposed to construct a discharge vessel by bending a long tube a number of times into respective U-shapes. This, however, require bending over a fairly large radius. Due to the wide radius it is possible to coat the inside of the tube with fluorescent material before bending.
The referenced Albrecht et al U.S. Pat. No. 4,481,442 describes a fluorescent lamp having a unitary discharge vessel in which the diameter of the discharge vessel in the region of the U-bend is larger than in the central portion of the connecting or base region of the U of the bent structure. This arrangement provides for the necessary cold spots to obtain optimum mercury vapor pressure, without formation of dark zones. Discharge vessels bent in accordance with this structure can be uniformly coated with fluorescent material only after bending. No difficulties are experienced in such a discharge vessel if a single U-shaped structure is provided; if, however, a multiple U-bent structure, with connected portions and the cross elements of the U at the opposite ends of the legs, is to be formed, complex and expensive procedures are necessary to provide for essentially uniform coating of the interior of the tube with fluorescent powder after formation of the tube structure.
It has been found that starting of a fluorescent lamp formed of a plurality of U-shaped elements may cause problems unless a more complex starting circuitry and ballast equipment is used than customary with only a two-leg U structure. This is true even for lamps operated directly from a 220 V alternating current supply; when operated with lower voltage power supply, for example 110 V nominal, obtaining sufficient ignition voltage becomes difficult. The temperature range of operation of accessory equipment, likewise, becomes more critical since the lamp voltage required for ignition or firing increases if temperatures deviate substantially from "room temperature" in both the direction towards high temperature as well as towards low temperature.